1. Field of the Invention
This invention relates to a method and apparatus, for charging and testing rechargeable batteries, especially lead-acid batteries of any capacity and voltage. More particularly, this invention relates to such a method and apparatus by which the characteristics of the battery, such as fault conditions are diagnosed in detail, and in which charging characteristics are automatically adjusted to suit the battery characteristics and the battery is charged with maximum efficiency and speed.
2. Prior Art
Methods and apparatus for determining the true capacity, nominal voltage, state of charge, defect, gassing, charge acceptance, and other characteristics of the battery helps to charge it efficiently, quickly and safely. Manual determination of these parameters is expensive and time consuming. With the introduction of microprocessor based autonomous chargers these operating characteristics are determined automatically and the battery is charged under optimum conditions.
Typically, the industrial lead-acid batteries have charge capacities of hundreds of ampere-hours. For maximum utilization, these batteries should be recharged quickly for the next use cycle. To prevent damage to the battery, it should be recharged soon after its use. The battery charger should be simple to operate, and should warn the user of battery defects and when to change the battery. It is desired that defective and unsafe conditions of the battery during charge should be indicated.
Some early commercial chargers employed constant charging current for a preset duration. Depending on the charge time available (the time the battery should be ready for use) and the capacity, the operator sets the charging current level. The efficiency of this type of charger is low, since state of charge (SOC) is not taken into account. Even with low initial SOC, the battery may evolve gas significantly in the last quarter of the charging cycle.
Some other chargers use constant voltage charging techniques for a specified time selecting a charge voltage which is lower than the gas voltage. This method may often lead to undercharging. Theoreticaly it takes an infinite time to charge, without gassing, a battery completely by this method since the charge current decreases asymptotically.
A charger from Westinghouse Devenset Rectifier of England employs another variation of this technique. The battery is supplied with a charging current until the battery voltage reaches a predetermined level corresponding to the gas evolution voltage (Vgas). The charging is continued from this stage by a timer for a specified period, followed by an equalizing charge. The battery is then placed under trickle charge to compensate for the open circuit self-discharge loss. The energy loss during the timer controlled charge period is still considerable and detrimental to the battery.
The charger introduced by Oldham/Harmer & Simmons of England passes a charging current to the battery until the voltage rises to the gas evolution voltage. The charger then alternates between a measuring cycle and a charging cycle. In the measuring cycle, the charge current is measured while the battery is charged under the constant voltage mode corresponding to the gas voltage. The charge is terminated when the currents in two successive measuring cycles are equal.
A charger employing periodic discharge pulses during the charge regime has been commercialized by Christie Electric Corp. The state of charge is derived from the current during the discharge pulse. This charger has been designed for small low capacity batteries.
The prior art has also described chargers using computers/microprocessors to perform analytical and control functions. One of the earliest chargers of this type analyzes voltage-current (I-V) characteristics during an applied current ramp to the battery. The I-V data is determined for each cell in the battery and compared with the average of all the cells. If any cell exhibits significantly different characteristics, the battery is diagnosed as defective. However, for practical purposes the cells in batteries are often inaccessible.
Another charger of this class uses the slope of the voltage current curve, obtained from the I-V characteristics as described above, to determine the state of charge of the battery. This is accomplished by comparing the above slope with those of average I-V characteristics of similar batteries at various charge levels (SOCs).
Yet another microprocessor based charger that uses I-V characteristics of current ramping test cycle has been proposed in EP Nos. 067589 and 067590. The I-V characteristics of the batteries of different capacities (within a narrow limit) and states of charge are stored in memory. They are compared with that of the battery being charged to determine SOC. If no match is found, the charger assumes a fault condition and calls for the attention of the operator. The battery is charged until the I-V characteristics are almost the same in successive test cycles.
All the chargers proposed in the state of the art are limited to batteries of certain nominal voltage and capacity within a narrow range. The fault-detecting diagnostics are also limited. For example, mismatched cells and soft-shorted cells are not signaled separately. Clearly, there is a need for a charger that can automatically identify the operating characteristics of the battery, detect fault conditions, and carry out charging with high efficiency and speed.